Fabry-Perot interferometers are used as optical filters and in spectroscopic sensors, for example. A Fabry-Perot interferometer is based on parallel mirrors, whereby a Fabry-Perot cavity is formed into a gap between the mirrors. The pass band wavelength of a Fabry-Perot interferometer can be controlled by adjusting the distance between the mirrors i.e. the width of the gap. It is common to use micromechanical technology for producing Fabry-Perot interferometers. Such a solution is described e.g. in patent document FI95838.
One such prior art structure of a micromechanical interferometer usually includes layers of silicon and silicon oxide, wherein mirrors of the interferometer have silicon oxide layer(s) between silicon layers. A movable mirror is provided by removing a sacrificial layer, which has initially been formed between two mirror layers. The sacrificial layer may be e.g., silicon dioxide, which can be removed by etching with hydrofluoric acid (HF), for example. In order to allow the etching substance to reach the sacrificial layer, holes are provided in the movable mirror. The position of a moveable mirror is controlled by applying voltage to electrodes, which are included in the mirror structures.
The micromechanical production technology allows series production of interferometers. However, there are some disadvantages related with the prior art solutions for production of interferometers and the interferometer components.
In order to prevent the silicon oxide layers of the mirrors to be etched during the etching of the sacrificial layer, silicon layers are provided between the sacrificial layer and the silicon oxide layers of the mirrors. However, when the sacrificial layer is etched through the holes of the movable mirror the silicon oxide layers may also become etched at the edges of the holes. In order to prevent the silicon oxide layer from becoming etched in the etching process of the sacrificial layer the silicon oxide layer can be removed by patterning around the areas of the holes before the upper layers are deposited. This way a silicon layer forms the edges of the through-holes. The removed area of silicon dioxide may have e.g., three times larger diameter size compared to the diameter of a hole. However, this causes that the released mirror to be not evenly thick at all locations, and the mirror has an inhomogeneous structure. While the mirror inevitably has a tensile stress, this will further cause bending of the mirror, which degrades the performance of the mirror. Providing the holes also requires several patterning/etching phases since the silicon dioxide layers must be patterned and etched separately.
The silicon layers at the surfaces of the mirrors also tend to have small holes, i.e. pinholes. During the etching of the sacrificial layer, it is therefore possible that the etching agent, such as HF, enters through the pinholes into the silicon oxide layers of the mirrors. If these layers are etched, the structure of the mirrors is deteriorated.
The density of the pinholes in the silicon layers is dependent on the roughness of the surface of the silicon layer. In order to minimize the density of pinholes, the degree of roughness of the silicon layers is made as low as possible. However, when the surfaces of the mirrors are smooth, there is an increased risk of the mirrors sticking to each other if they touch each other. The mirrors may touch each other during the use or transportation, for example. For example, instantaneous overvoltage at the control circuit of the component or high humidity may cause the mirrors to stick permanently to each other and thus cause the component to become non-functional.
A further problem relates to removing the sacrificial layer between the mirrors, In prior art processes the removal is a separate process which must be made before the interferometers can be cut out from the wafers and encapsulated. Such a separate process increases the complexity of the production process. Also, the cutting, encapsulating and transportation of the interferometers require special handling because of the movable, released mirror. A released mirror is sensitive to environmental stress, such as changes of temperature or humidity, contamination, and the like.
As a result of these disadvantages, the yield of interferometers in the production may be low, and reliability of the produced interferometers possibly does not reach a required level.